The present invention relates to a method for central nervous system (CNS) monitoring, and more specifically to a method of positioning electrodes in an electrode array comprising three electrodes for monitoring central nervous system (CNS), ie electroencephalography (EEG) and frontal electromyography (FEMG) signals from the forehead of a patient's head. The invention also relates to a method of sensing pain reactions of a patient.
Electroencephalography (EEG) is a well-established method for assessing the brain function by picking up the weak signals generated in the brain with electrodes on the skull surface. To obtain the signals, multiple electrodes are placed on the scalp of a patient in accordance with a recognized protocol. EEG has been in wide use for decades in basic research of the neural system of brain as well as clinically in diagnosis of various neurophysiological disorders.
In a traditional EEG measurement electrodes are attached following the standard 10–20 system. Said system has been used by neurophysiologists for decades to record EEG and to find pathological EEG changes. The system however requires cumbersome attachment of multiple electrodes, especially, when the electrodes are attached in the hair environment.
One of the special applications for EEG which has received attention to during the 1990's is use of a processed EEG signal for objective quantification of the amount of brain activity for the purpose of determining the level of consciousness of a patient. In its simplest form, the usage of EEG allows for the automatic detection of the alertness of an individual, ie. if he or she is awake or asleep. This has become a significant issue, both scientifically and commercially, in the context of measuring the depth of unconsciousness induced by anesthesia during surgery. Modern anesthesia practices use a sophisticated balanced anesthesia technique with a combination of drugs for maintaining adequate hypnosis, analgesia, muscle relaxation, and suppression of the autonomic nervous system. The need for a reliable system for monitoring of the adequacy of the anesthesia is based on both safety and economical concerns. An anesthesia dose which is too light can, in the worst case, cause the patient wake up in the middle of the operation and create a highly traumatic experience both for the patient and for the personnel administering the anesthesia. At the opposite extreme, the administration of too much anesthesia generates increased costs due to the excessive use of anesthesia drugs and the time needed to administer the drugs. Over dosage of the anesthesia drugs also affects the quality and length of the post-operative period immediately after the operation and the time required for any long-term post-operative care.
In the anesthesia and the intensive care environment said 10–20 system is very rarely used. This is because these environments are already crowded by many other measuring systems, such as blood pressure, ECG, inspired and expired gas measurements. The additional labour-consuming measuring system would take too much time and effort from the care personnel. There is even though need for central nervous system monitoring in these areas. The consciousness level of the patient is varied in both of said environments and till today there has not been a practical method for monitoring the level of consciousness in the anesthesia and the intensive care environment.
As told before in the anesthesia environment patient is anesthetized with hypnotic, analgesic and neuromuscular blocking agents. The neuromuscular blocking agents, given in a certain extent block the neuromuscular junction and the patient looses ability to move herself/himself. This can create a situation where patient feels pain but cannot communicate. Without central nervous system monitoring there is a risk of giving too little or too much anesthetics. If too little hypnotic drugs is given to the patient she/he could awake during operation, which could cause traumatic experience especially for the patient and also for the personnel. On the other hand over dosage of hypnotic drugs affects the quality and length of the post-operative period.
The above mentioned reasons have generated commercial efforts to develop EEG devices to said environments during the past ten years. The main requirements for such monitoring can be described by the following features, ease of use, reliability and good quality. The efforts in this area have concentrated into reliable and easy electrodes as well as to good quality signal processing.
A significant advancement in making the EEG-based measurement of the adequacy of anesthesia an easy-to-use, routine was a finding based on Positron Emission Tomography (PET) that determined that the effects of the anesthetic drugs on the brain are global in nature. This means that for many applications it is enough to measure the forebrain or frontal cortex EEG from the forehead of the patient. The forehead is both an easy to access and is hairless location on the patient. Electrodes placed with an appropriate spacing between the electrodes on the forehead can pick up an adequate signal originating from the anterior cortex in the brain.
Since the Positron Emission Tomography (PET) studies have shown that the anesthesia effect is a global phenomena in the brain, the sensor development efforts have concentrated on the hairless frontal area of the head. The first commercial sensor for this application area was developed by the company Aspect Medical Systems, Inc. U.S. Pat. No. 6,032,064 can be mentioned as an example of the art describing the sensor developed by Aspect Medical Systems, Inc. The company mentioned above also has patented many electrode configurations relating to placement of the electrodes on frontal and temple areas of the patient's head. Reference is made here to U.S. Pat. No. 6,394,953.
While the foregoing has discussed the use of EEG signals, it is also desirable to obtain frontal electromyographic (FEMG) signals arising from the forehead of the patient. The frontalis muscle is the first indicator of approaching consciousness. When this muscle activity is sensed by appropriately placed electrodes it provides an early indication that the patient is emerging from anesthesia. Similarly these electrodes can sense pain reactions originating from this same muscle activity when the anesthesia is not adequate, for example because of inadequate analgesia. So the FEMG signals give an early warning of arousal and may also indicate inadequate analgesia.